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class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-怎么理解回流跟重绘-什么场景下会触发"><a href="#面试官-怎么理解回流跟重绘-什么场景下会触发" class="header-anchor">#</a> 面试官：怎么理解回流跟重绘？什么场景下会触发？</h1> <p><img src="https://static.vue-js.com/1ed5d340-9cdc-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h2 id="一、是什么"><a href="#一、是什么" class="header-anchor">#</a> 一、是什么</h2> <p>在<code>HTML</code>中，每个元素都可以理解成一个盒子，在浏览器解析过程中，会涉及到回流与重绘：</p> <ul><li><p>回流：布局引擎会根据各种样式计算每个盒子在页面上的大小与位置</p></li> <li><p>重绘：当计算好盒模型的位置、大小及其他属性后，浏览器根据每个盒子特性进行绘制</p></li></ul> <p>具体的浏览器解析渲染机制如下所示：</p> <p><img src="https://static.vue-js.com/2b56a950-9cdc-11eb-ab90-d9ae814b240d.png" alt=""></p> <ul><li><p>解析HTML，生成DOM树，解析CSS，生成CSSOM树</p></li> <li><p>将DOM树和CSSOM树结合，生成渲染树(Render Tree)</p></li> <li><p>Layout(回流):根据生成的渲染树，进行回流(Layout)，得到节点的几何信息（位置，大小）</p></li> <li><p>Painting(重绘):根据渲染树以及回流得到的几何信息，得到节点的绝对像素</p></li> <li><p>Display:将像素发送给GPU，展示在页面上</p></li></ul> <p>在页面初始渲染阶段，回流不可避免的触发，可以理解成页面一开始是空白的元素，后面添加了新的元素使页面布局发生改变</p> <p>当我们对 <code>DOM</code> 的修改引发了 <code>DOM</code>几何尺寸的变化（比如修改元素的宽、高或隐藏元素等）时，浏览器需要重新计算元素的几何属性，然后再将计算的结果绘制出来</p> <p>当我们对 <code>DOM</code>的修改导致了样式的变化（<code>color</code>或<code>background-color</code>），却并未影响其几何属性时，浏览器不需重新计算元素的几何属性、直接为该元素绘制新的样式，这里就仅仅触发了回流</p> <h2 id="二、如何触发"><a href="#二、如何触发" class="header-anchor">#</a> 二、如何触发</h2> <p>要想减少回流和重绘的次数，首先要了解回流和重绘是如何触发的</p> <h3 id="回流触发时机"><a href="#回流触发时机" class="header-anchor">#</a> 回流触发时机</h3> <p>回流这一阶段主要是计算节点的位置和几何信息，那么当页面布局和几何信息发生变化的时候，就需要回流，如下面情况：</p> <ul><li>添加或删除可见的DOM元素</li> <li>元素的位置发生变化</li> <li>元素的尺寸发生变化（包括外边距、内边框、边框大小、高度和宽度等）</li> <li>内容发生变化，比如文本变化或图片被另一个不同尺寸的图片所替代</li> <li>页面一开始渲染的时候（这避免不了）</li> <li>浏览器的窗口尺寸变化（因为回流是根据视口的大小来计算元素的位置和大小的）</li></ul> <p>还有一些容易被忽略的操作：获取一些特定属性的值</p> <blockquote><p>offsetTop、offsetLeft、 offsetWidth、offsetHeight、scrollTop、scrollLeft、scrollWidth、scrollHeight、clientTop、clientLeft、clientWidth、clientHeight</p></blockquote> <p>这些属性有一个共性，就是需要通过即时计算得到。因此浏览器为了获取这些值，也会进行回流</p> <p>除此还包括<code>getComputedStyle</code>方法，原理是一样的</p> <h3 id="重绘触发时机"><a href="#重绘触发时机" class="header-anchor">#</a> 重绘触发时机</h3> <p>触发回流一定会触发重绘</p> <p>可以把页面理解为一个黑板，黑板上有一朵画好的小花。现在我们要把这朵从左边移到了右边，那我们要先确定好右边的具体位置，画好形状（回流），再画上它原有的颜色（重绘）</p> <p>除此之外还有一些其他引起重绘行为：</p> <ul><li><p>颜色的修改</p></li> <li><p>文本方向的修改</p></li> <li><p>阴影的修改</p></li></ul> <h3 id="浏览器优化机制"><a href="#浏览器优化机制" class="header-anchor">#</a> 浏览器优化机制</h3> <p>由于每次重排都会造成额外的计算消耗，因此大多数浏览器都会通过队列化修改并批量执行来优化重排过程。浏览器会将修改操作放入到队列里，直到过了一段时间或者操作达到了一个阈值，才清空队列</p> <p>当你获取布局信息的操作的时候，会强制队列刷新，包括前面讲到的<code>offsetTop</code>等方法都会返回最新的数据</p> <p>因此浏览器不得不清空队列，触发回流重绘来返回正确的值</p> <h2 id="三、如何减少"><a href="#三、如何减少" class="header-anchor">#</a> 三、如何减少</h2> <p>我们了解了如何触发回流和重绘的场景，下面给出避免回流的经验：</p> <ul><li>如果想设定元素的样式，通过改变元素的 <code>class</code> 类名 (尽可能在 DOM 树的最里层)</li> <li>避免设置多项内联样式</li> <li>应用元素的动画，使用 <code>position</code> 属性的 <code>fixed</code> 值或 <code>absolute</code> 值(如前文示例所提)</li> <li>避免使用 <code>table</code> 布局，<code>table</code> 中每个元素的大小以及内容的改动，都会导致整个 <code>table</code> 的重新计算</li> <li>对于那些复杂的动画，对其设置 <code>position: fixed/absolute</code>，尽可能地使元素脱离文档流，从而减少对其他元素的影响</li> <li>使用css3硬件加速，可以让<code>transform</code>、<code>opacity</code>、<code>filters</code>这些动画不会引起回流重绘</li> <li>避免使用 CSS 的 <code>JavaScript</code> 表达式</li></ul> <p>在使用 <code>JavaScript</code> 动态插入多个节点时, 可以使用<code>DocumentFragment</code>. 创建后一次插入. 就能避免多次的渲染性能</p> <p>但有时候，我们会无可避免地进行回流或者重绘，我们可以更好使用它们</p> <p>例如，多次修改一个把元素布局的时候，我们很可能会如下操作</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> el <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'el'</span><span class="token punctuation">)</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span><span class="token number">10</span><span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    el<span class="token punctuation">.</span>style<span class="token punctuation">.</span>top  <span class="token operator">=</span> el<span class="token punctuation">.</span>offsetTop  <span class="token operator">+</span> <span class="token number">10</span> <span class="token operator">+</span> <span class="token string">&quot;px&quot;</span><span class="token punctuation">;</span>
    el<span class="token punctuation">.</span>style<span class="token punctuation">.</span>left <span class="token operator">=</span> el<span class="token punctuation">.</span>offsetLeft <span class="token operator">+</span> <span class="token number">10</span> <span class="token operator">+</span> <span class="token string">&quot;px&quot;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>每次循环都需要获取多次<code>offset</code>属性，比较糟糕，可以使用变量的形式缓存起来，待计算完毕再提交给浏览器发出重计算请求</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 缓存offsetLeft与offsetTop的值</span>
<span class="token keyword">const</span> el <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'el'</span><span class="token punctuation">)</span>
<span class="token keyword">let</span> offLeft <span class="token operator">=</span> el<span class="token punctuation">.</span>offsetLeft<span class="token punctuation">,</span> offTop <span class="token operator">=</span> el<span class="token punctuation">.</span>offsetTop

<span class="token comment">// 在JS层面进行计算</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span><span class="token number">10</span><span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  offLeft <span class="token operator">+=</span> <span class="token number">10</span>
  offTop  <span class="token operator">+=</span> <span class="token number">10</span>
<span class="token punctuation">}</span>

<span class="token comment">// 一次性将计算结果应用到DOM上</span>
el<span class="token punctuation">.</span>style<span class="token punctuation">.</span>left <span class="token operator">=</span> offLeft <span class="token operator">+</span> <span class="token string">&quot;px&quot;</span>
el<span class="token punctuation">.</span>style<span class="token punctuation">.</span>top <span class="token operator">=</span> offTop  <span class="token operator">+</span> <span class="token string">&quot;px&quot;</span>
</code></pre></div><p>我们还可避免改变样式，使用类名去合并样式</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> container <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'container'</span><span class="token punctuation">)</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>width <span class="token operator">=</span> <span class="token string">'100px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>height <span class="token operator">=</span> <span class="token string">'200px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>border <span class="token operator">=</span> <span class="token string">'10px solid red'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>color <span class="token operator">=</span> <span class="token string">'red'</span>
</code></pre></div><p>使用类名去合并样式</p> <div class="language-html extra-class"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>style</span><span class="token punctuation">&gt;</span></span><span class="token style"><span class="token language-css">
    <span class="token selector">.basic_style</span> <span class="token punctuation">{</span>
        <span class="token property">width</span><span class="token punctuation">:</span> 100px<span class="token punctuation">;</span>
        <span class="token property">height</span><span class="token punctuation">:</span> 200px<span class="token punctuation">;</span>
        <span class="token property">border</span><span class="token punctuation">:</span> 10px solid red<span class="token punctuation">;</span>
        <span class="token property">color</span><span class="token punctuation">:</span> red<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>style</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    <span class="token keyword">const</span> container <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'container'</span><span class="token punctuation">)</span>
    container<span class="token punctuation">.</span>classList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">'basic_style'</span><span class="token punctuation">)</span>
</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><p>前者每次单独操作，都去触发一次渲染树更改（新浏览器不会），</p> <p>都去触发一次渲染树更改，从而导致相应的回流与重绘过程</p> <p>合并之后，等于我们将所有的更改一次性发出</p> <p>我们还可以通过通过设置元素属性<code>display: none</code>，将其从页面上去掉，然后再进行后续操作，这些后续操作也不会触发回流与重绘，这个过程称为离线操作</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> container <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'container'</span><span class="token punctuation">)</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>width <span class="token operator">=</span> <span class="token string">'100px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>height <span class="token operator">=</span> <span class="token string">'200px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>border <span class="token operator">=</span> <span class="token string">'10px solid red'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>color <span class="token operator">=</span> <span class="token string">'red'</span>
</code></pre></div><p>离线操作后</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> container <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">getElementById</span><span class="token punctuation">(</span><span class="token string">'container'</span><span class="token punctuation">)</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>display <span class="token operator">=</span> <span class="token string">'none'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>width <span class="token operator">=</span> <span class="token string">'100px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>height <span class="token operator">=</span> <span class="token string">'200px'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>border <span class="token operator">=</span> <span class="token string">'10px solid red'</span>
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>color <span class="token operator">=</span> <span class="token string">'red'</span>
<span class="token operator">...</span>（省略了许多类似的后续操作）
container<span class="token punctuation">.</span>style<span class="token punctuation">.</span>display <span class="token operator">=</span> <span class="token string">'block'</span>
</code></pre></div><h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li><p>https://juejin.cn/post/6844903942137053192</p></li> <li><p>https://segmentfault.com/a/1190000017329980</p></li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <!----> </main></div><div class="global-ui"><!----></div></div>
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